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Lab 12 | CSCI 206 Computer Organization & Programming

Lab 12

Bomb Lab

The nefarious Dr. Evil has planted a slew of “binary bombs” on our class machines. A binary bomb is a program that consists of a sequence of phases. Each phase expects you to type a particular string on stdin. If you type the correct string, then the phase is defused and the bomb proceeds to the next phase. Otherwise, the bomb explodes by printing “BOOM!!!” and then terminating. The bomb is defused when every phase has been defused. There are too many bombs for us to deal with, so we are giving each student a bomb to defuse. Your mission, which you have no choice but to accept, is to defuse your bomb before the due date. Good luck, and welcome to the bomb squad!

Step 1: Get your bomb

You will receive your own personal bomb via email (to your bucknell.edu address) with the subject line “CSCI206 – Bomb Lab” the  actual bomb is in the attached bomb.tar archive. (a tar file is roughly the Linux equivalent of a zip file). All bombs have a unique solution. Using a lab computer, create a folder for this lab (~/cs206/Labs/Lab12) and copy the bomb.tar file to your Lab12 working folder. From the command prompt folder, use the following command to unpack the contents (assuming you already cd’ed into that folder):

$ tar -xvf bomb.tar

You will now have three new files:

  • README: Identifies the owner of this particular bomb and gives you your unique Bomb number for grading purposes (do not trade bombs, we will grade based on the assigned bomb number only!)
  • bomb: The executable binary bomb
  • bomb.c: Source file with the bomb’s main routine and a friendly greeting from Dr. Evil.

You can now safely remove the tar file.

Step 2: Defuse your bomb

Your job for this lab is to defuse your bomb.

You must do the assignment on the mips.bucknell.edu machine since it is compiled for the mips architecture and won’t even run on an Intel machine.

You can use many tools to help you defuse your bomb. Please look at the hints section for some tips and ideas. The best way is to use your favorite debugger (gdb) to step through the disassembled binary.

Each time your bomb explodes it notifies the bomblab server, and you lose 1/2 point (up to a max of 20 points) in the final score for the lab. So there are consequences to exploding the bomb. You must be careful!

The first four phases are worth 10 points each. Phases 5 and 6 are a little more difficult, so they are worth 15 points each. So the maximum score you can get is 70 points.

Although phases get progressively harder to defuse, the expertise you gain as you move from phase to phase should offset this difficulty. However, the last phase will challenge even the best students, so please don’t wait until the last minute to start.

The bomb ignores blank input lines. If you run your bomb with a command line argument, for example,

$./bomb sol.txt

then it will read the input lines from sol.txt until it reaches EOF (end of file), and then switch over to stdin. In a moment of weakness, Dr. Evil added this feature so you don’t have to keep retyping the solutions to phases you have already defused.

To avoid accidentally detonating the bomb, you will need to learn how to single-step through the assembly code and how to set breakpoints. You will also need to learn how to inspect both the registers and the memory states. One of the nice side-effects of doing the lab is that you will get very good at using a debugger. This is a crucial skill that will pay big dividends the rest of your career.

Scoreboard

We have a nifty server that will track your progress. The scoreboard is available from the campus network here: http://mips.bucknell.edu:15213/scoreboard. Your progress will not be visible until your bomb communicates back to the server. The bomb will communicate back to the server when a stage is defused or the bomb explodes. Note to minimize the load on the server, this page is statically generated every 90 seconds, so it may take up to 90 seconds for recent activity to appear.

Submit

Submit the text file sol.txt in your git repo. This file will be run with your given executable to determine your final score following the standard scoring rules (i.e. it will match your score on the scoreboard).

Hints: You should really read this

There are many ways of defusing your bomb. You can examine it in great detail without ever running the program, and figure out exactly what it does. This is a useful technique, but it not always easy to do. You can also run it under a debugger, watch what it does step by step, and use this information to defuse it. This is probably the fastest way of defusing it.

We do make one request, please do not use brute force! You could write a program that will try every possible key to find the right one. But this is no good for several reasons:

  • You lose 1/2 point (up to a max of 20 points) every time you guess incorrectly and the bomb explodes.
  • Every time you guess wrong, a message is sent to the bomblab server. You could very quickly saturate the network with these messages, and cause the system administrators to revoke your computer access.
  • You might even crash the bomblab server,  which would be a pain for everyone since they would be unable to make progress on their own bomb until the server is restarted.
  • We haven’t told you how long the strings are, nor have we told you what characters are in them. Even if you made the (incorrect) assumptions that they all are less than 80 characters long and only contain letters, then you will have 26^80 guesses for each phase. This will take a very long time to run, and you will not get the answer before the assignment is due.

There are many tools which are designed to help you figure out both how programs work, and what is wrong when they don’t work. Here is a list of some of the tools you may find useful in analyzing your bomb, and hints on how to use them.

  • gdb

The GNU debugger, this is a command line debugger tool available on virtually every platform. You can trace through a program line by line, examine memory and registers, look at both the source code and assembly code (we are not giving you the source code for most of your bomb), set breakpoints, set memory watch points, and write scripts.

Here is a handy list of gdb commands you can print out and use as a reference.

    • To keep the bomb from blowing up every time you type in a wrong input, you’ll want to learn how (and where) to set breakpoints.
    •  For online documentation, type “help” at the gdb command prompt, or type “man gdb”, or “info gdb” at a Unix prompt. Some people also like to run gdb under gdb-mode in emacs.
  • objdump -t

This will print out the bomb’s symbol table. The symbol table includes the names of all functions and global variables in the bomb, the names of all the functions the bomb calls, and their addresses. You may learn something by looking at the function names!

  • objdump -d

Use this to disassemble all of the code in the bomb. You can also just look at individual functions. Reading the assembler code can tell you how the bomb works. Although objdump -d gives you a lot of information, it doesn’t tell you the whole story. Calls to system-level functions are displayed in a cryptic form.  To determine what is being called, you would need to disassemble within gdb.

Acknowledgement

This lab was developed by R. Bryant and D. O’Hallaron and is published  on the CS:APP website. All Evilness herein is fully their doing.

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